Wood Foundation Basics and TechniquesUpdated: Jun. 30, 2017
If you engineer and assemble wood foundations properly, they're extremely durable
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Why wood foundations are durable
If you're considering a wood basement, don't worry about durability and strength. If in doubt, it's best to get an opinion from someone who's built a few dozen and had to guarantee their performance—me! I'm positively gleeful over the 20 trouble-free years we've had with ours at home and have yet to hear a single complaint from any of the dozens of customers I've built them for. The clients I still hear from love the advantages wood foundations have to offer. You can build them yourself, they're dry and warm, and they're easy to finish (inside and out). And because they're simple wood frame construction, they're easy to alter even after the basement's in. In fact, I added six windows and doors in my walkout basement three years after the house was finished.
You won't find too many unhappy wood foundation owners as long as the installation is done properly. Successful wood foundations depend on using the proper wood, good drainage and waterproofing. Most people considering a wood foundation are concerned about rot and strength. “Foundation-grade” wood used in foundations is treated by steam-impregnating it with a chemical called CCA (chromated copper arsenate) at a concentration of at least .6 lbs. of chemical per cubic foot of wood so that the chemical penetrates deep into the core of the wood. The copper part of the compound is toxic to fungus, mold and bacteria, while the arsenate is toxic to pests like carpenter ants and termites.
Long-term durability isn't an issue with a sound wood foundation. That's been well documented by U.S. Dept. of Agriculture's Forest Service testing. Proper construction techniques are everything. When wood foundations fail, it's always a case of poor construction techniques. The key elements are properly sized sheathing and framing, secure floor tie-ins and proper drainage.
Strength is a matter of engineering
Strength is simply a matter of following the guidelines laid out in the wood foundation manual put out by the Southern Pine Council. Framing member widths and spacing as well as sheathing thickness depend mostly on backfill height. The deeper into the ground, the stronger the wall needs to be. In our example, 2x8s are spaced every 16 in. with 5/8-in. sheathing, with the wall resting on 2x10s, which in turn rest on an 8-in. thick gravel footing. The concrete floor resists ground pressure at the bottom, while the top is anchored to the floor system above with joist hangers and clips. This design is acceptable for most normal-height backfill scenarios. Any and all water will filter through the gravel backfill, footings and under-slab fill where it's collected by a perforated sump basket for pumping away from the house, or drained to daylight if your home's on a hill.
Hire a wood foundation built and the cost is roughly equivalent to concrete block and a little cheaper than poured walls. If you build it yourself, you'll get it for half the price of a conventionally built masonry basement. But keep in mind that finishing is much easier and cheaper because you're finishing a stud wall.
If you plan to install a wood foundation, get help with the design and construction first by ordering a booklet from The Southern Pine Council called “Permanent Wood Foundations.” Search “Southern Pine Council” for more information.
Wood Foundation Details
Wood foundations require foundation grade pressure treated wood, studs sized for soil pressure, the proper fasteners, solid gravel footings, damp-proofing and good drainage.